Small optical disk drive

ABSTRACT

The invention relates to an optical disk drive, which can be built very small. In order to solve cooling problems and collecting light reflected from the disk with maximum efficiency an optical disk drive is proposed comprising:—a laser diode for generating a laser beam, said laser diode being located at a fixed position in the drive,—an objective for focussing said laser beam on an optical disk,—a light guide for guiding said laser beam from said laser diode to said objective,—a detector for detecting light reflected from the optical disk, and—a movable swing arm for carrying said objective and said detector, said detector being arranged close to said objective such that the light reflected from the optical disk through said objective is detected.

The invention relates to an optical disk drive and to a detector for usein an optical disk drive.

One of the challenges in making a small form factor optical disk drive,in particular having a small overall size or being thin for use in anotebook, is the integration of optics, mechanics and electronics makingup the optical pick-up head. A multitude of solutions are alreadysuggested. A high density data storage system with specifically shapedoptical fibre as optical read/write head is, for example, disclosed inU.S. Pat. No. 6,069,861. Therein, an optics of a read/write systemincludes a stationary optical device for condensing a light beam andleading said light beam into an optical fibre. A distal end of theoptical fibre is moveable and with a tapered configuration thereof forfocusing the light emerging from the optical fibre into a beam spot onan optical recording medium. A detector detects through the opticalfibre light reflected back from the recording medium.

One of the major problems to solve in integrating a laser diode, inparticular a blue laser diode chip, with the rest of the optics andelectronics, is the heat dissipation of the laser diode and its drivingcircuitry. If the optical head is positioned on a very small, low massswing arm, cooling properties are very poor. According to U.S. Pat. No.6,069,861 the laser for generating the laser beam as well as the photodiode for receiving reflected light are arranged stationary in theoptical drive, but not on the swing arm. Thus, the optical fibre is usedfor guiding light from the laser to the recording medium as well as forguiding reflected light from the recording medium to the photo diode.This considerably reduces efficiency of the overall system and may causesignal detection errors.

It is therefore an object of the present invention to provide an opticaldisk drive, comprising a swing arm, which overcomes the above describedproblems, in particular which has a higher efficiency, avoids signaldetection errors, can be built as compact as possible and solves theheating problem.

These objects are achieved by an optical disk drive as claimed in claim1 comprising:

-   -   a laser diode for generating a laser beam, said laser diode        being located at a fixed position in the drive,    -   an objective for focussing said laser beam on an optical disk,    -   a light guide for guiding said laser beam from said laser diode        to said objective,    -   a detector for detecting light reflected from the optical disk,        and    -   a moveable swing arm for carrying said objective and said        detector, said detector being arranged close to said objective        such that the light reflected from the optical disk through said        objective is detected.

The invention is based on the idea to put a laser diode at a fixedposition in the disk drive and transport the light to the optical headusing a light guide, preferably an optical fibre. Detection of radialand focus tracking error signals and HF signals can be done in theoptical head itself which is positioned on the moveable swing arm. Saidoptical head includes an objective for focusing a laser beam on theoptical disk and a detector for detecting light reflected from theoptical disk. Since according to the invention the detector is arrangedclose to said objective, particularly directly underneath the objectiveand next to the fibre exit, a light reflected from the optical disk canbe collected with maximum efficiency and optical head functionality canbe realised as compact as possible. Since the light guide is only usedfor transporting the laser beam from the laser diode to pupilinformation upon reflection at the disk is preserved, facilitatingtracking and data readout. The heat dissipation from the laser diode andassociated driver electronics can be easily handled by using appropriatecooling means near the laser diode and the driver electronics sincethese elements are arranged at a fixed position in the optical diskdrive but not on the moveable swing arm.

Preferred embodiments in the optical disk drive are claimed in thedependent claims. According to a first preferred embodiment the detectoris arranged around the exit of the light guide emitting the laser beam.Thus, the same objective can be used for focusing the laser beam on theoptical disk and for focusing the light reflected from the optical diskon the detector. In this embodiment a split detector is preferably usedhaving at least two detector portions, the exit of the light guide beingarranged in a slit between said detector portions. The light guide exitis thus located exactly in the middle of both detector halves. By givingboth detector halves a slight offset such that the distance from theoptical disk to the surface of said detector halves is different, thisdetector can be used for spot-size detection without the use of anyadditional optics.

According to another preferred embodiment the detector is arranged nextto the exit of the light guide so that no or not much light reflectedfrom the optical disk is reflected back into the light guide. In orderto deflect the reflected light towards the detector, a quarter waveplate and a hologram are preferably arranged between the objective andthe optical disk, which hologram is preferably replicated on top of saidobjective or on said quarter wave plate.

According to a further aspect of the invention the detector is adaptedfor detection of radial and focus error tracking error signals(push-pull signals) and of HF data The detection of the push-pullsignals can be realized by an appropriate subdivision of the detector,while the detection of the HF data can be realized by adding all thesignals detected by the detector segments.

Preferably an optical fibre, in particular a single mode fibre, is usedas said light guide. However, instead of optical fibres the use ofintegrated waveguides might be considered.

The invention can preferably be applied in a small sized optical diskdrive wherein the laser diode is adapted for generating a blue laserbeam. Further, the objective is preferably mounted on a focus controlelement, in particular a leaf spring, for focus control and focus andradial tracking means are provided, in particular integrated planarcoils, mounted on the swing arm for focussing and radial tracking of thelaser beam.

The invention also relates to a detector for use in an optical diskdrive as described above for detecting light reflected from the opticaldisk, said detector being a split detector having at least two detectorportions.

The invention will now be explained in more details with reference tothe drawings, in which

FIG. 1A shows a first embodiment of an optical disk drive according tothe invention,

FIG. 1B shows a first embodiment of a detector used therein,

FIG. 1C shows a top view on the detector according to FIG. 1B,

FIG. 2A shows a second embodiment of an optical disk drive according tothe invention,

FIG. 2B shows a second embodiment of a detector,

FIG. 2C shows a top view on the detector according to FIG. 2B,

FIG. 3A/3B show a third embodiment of an optical disk drive according tothe invention illustrating focus and radial tracking means.

FIG. 1A shows the main elements of a first embodiment of an optical diskdrive according to the present invention. An optical disk 1, e.g. a CDor a DVD, is rotated around a spindle 2 by a spindle motor 3. A laserdiode 4 is placed at a fixed position in the drive and the laser lightgenerated by said laser diode is an objective lens 5 by means of anoptical fibre 6. This fibre 6 is preferably a single mode fibre, havingpreferably a core diameter smaller than 1 μm, in order to ensure perfectTEM 00 output mode stability at the output. Furthermore the fibre 6 isused in this way as a beam shaping device since the laser beam 7 at theoutput of the fibre 6 is perfectly circular shaped. The diverging laserbeam is focused by the objective 5, which is a finite conjugate both atimage and object space, on the optical disk 1.

The light reflected from the disk 1 is imaged by the same objective lens5 on top of a split detector 8. The image size is approximately thenumerical aperture of the objective divided by the numerical aperture ofthe fibre output multiplied by the imaged spot-size on the disk, thenumerical aperture of the fibre output being approximately 0.05. Asshown in FIG. 1B the fibre exit 60 is located exactly in the middle oftwo detector halves 81, 82. Since the imaged spot by the objective lens5 can be as large as 20 μm, the whole detector pupil can be filled,detecting push-pull and HF data. By giving both detector halves 81, 82 aslight offset, e.g. being 20-50 μm in z-height, e.g. in a directionperpendicular to the surface of the optical disk 1, this geometry can beused as spot-size detection without the use of additional optics. FIG.1C shows a top view onto the objective lens 5 and the detector 8 as wellas the fibre exit 60.

The objective lens 5 is mounted on a thin leaf spring 9 that can beactuated in vertical direction, i.e. in the direction perpendicular tothe surface of the optical disk 1, for focus control. Below theobjective lens the detector 8 is located on a low mass swing arm 10 towhich the leaf spring 9 is mechanically connected. For the purpose offocus control the leaf spring 9 contains a planar coil 11 that ispositioned in a homogeneous static magnetic field which is generated bytwo magnets 12 placed at the end of the swing arm 10. During actuating,due to tilt of the leaf spring 9, the objective lens 5 must tolerate acertain amount of field.

A disadvantage of the geometry shown in FIG. 1A is that laser lightreflected back from the optical disk might be fedback via the opticalfibre 6 into the laser diode 4. Although this is only a minor fractionof the power emitted by the laser diode 4, this feedback may give riseto a signal to noise ratio performance decrease. Another embodiment ofan optical disk drive according to the invention which is improved inthis respect is shown in FIG. 2A. Therein the diverging laser beam 71 isfocused by an objective 5 through a quarter wave plate 13 on the opticaldisk 1. The light 72 reflected from the disk is imaged by the sameobjective lens 5 on top of the split detector 8.

Different from the embodiment shown in FIG. 1A the detector 8 ispositioned next to the fibre exit 60 as shown in FIG. 2B. An additionalhologram (not shown) and the quarter wave plate 13 deflect the reflectedlaser beam 72 towards the detector 8. The hologram can be replicated ontop of the objective lens 5 or on the quarter wave plate 13. Again, thewhole detector pupil can be filled, detecting push-pull and HF data. Thedetector is a split detector having two detector halves 81, 82 havingdifferent distances to the surface of the optical disk 1. A top view onthe detector 8, the objective lens 5 and the fibre exit 60 is shown inFIG. 2C.

The advantages of this geometry compared to the embodiment shown in FIG.1A are less or even no optical feedback into the laser diode 4 andeasier detector placement since there is no interference with theoptical fibre. However, polarising optics are preferably required, i.e.a quarter wave plate is used for rotation of the polarisation state.This is necessary to ensure selective use of the hologram. Only in thelight path from the optical disk to the detector the hologram shall beactive. Furthermore, the polarisation and the output of the opticalfibre needs to be controlled, e.g. by use of a polarisation modeconserving fibre.

FIG. 3A shows a top view on a leaf spring 9 used in the optical diskdrive as shown in FIG. 2A. On the front portion 90 of the leaf springthere are three integrated planar coils, one planar coil 110 for focuscontrol F (see FIG. 3C where the directions of movement for focuscontrol F are indicated) and two planar coils 111, 112 for radialcontrol R. As indicated in FIG. 3B showing a side view of the frontportion 90 the three coils 110, 111, 112 are placed in a fixedhomogeneous magnetic field generated by coils 12 placed on the swing arm10.

According to the invention many parts of the optical engine includingthe laser diode and its driving circuitry can effectively be cooledwithout the need to provide intensive cooling elements on the swing armitself. Further, since the detector can be built directly underneath theobjective lens and next to the fibre exit, this arrangement allowscollection of the light reflected from the disk with maximum efficiencyand realisation of an optical head functionality as compact as possible.A detector geometry for focus and radial tracking can easily beincorporated. Thus, the invention allows a realisation of a small formfactor optical disk drive. Instead of optical fibres also the use ofintegrated waveguides might be considered, using an integrated 45degrees mirror for directing the light to the objective; alternativelyV-groove fiber couplers, comprising etched end mirrors also facilitatethe coupling of light out of an optical fibre into the objective lens.

1. Optical disk drive comprising: a laser diode for generating a laserbeam, said laser diode being located at a fixed position in the drive,an objective for focussing said laser beam on an optical disk, a lightguide for guiding said laser beam from said laser diode to saidobjective, a detector for detecting light reflected from the opticaldisk, and a movable swing arm for carrying said objective and saiddetector, said detector being arranged close to said objective such thatthe light reflected from the optical disk through said objective isdetected.
 2. Optical disk drive according to claim 1, wherein saiddetector is arranged around the exit of the light guide emitting thelaser beam.
 3. Optical disk drive according to claim 2, wherein saiddetector is a split detector having at least two detector portions, theexit of the light guide being arranged in a slit between said detectorportions.
 4. Optical disk drive according to claim 3, wherein saiddetector portions have different heights such that the distance from theoptical disk to the surface of said detector portions is different. 5.Optical disk drive according to claim 1, wherein said detector isarranged next to the exit of said light guide.
 6. Optical disk driveaccording to claim 5, further comprising a quarter wave plate and ahologram arranged between said objective and the optical disk fordeflecting the light reflected from the optical disk onto said detector.7. Optical disk drive according to claim 6, wherein said hologram isreplicated on top of said objective or on said quarter wave plate. 8.Optical disk drive according to claim 1, wherein said detector isadapted for detection of a radial and focus error tracking error signalsand of HF data.
 9. Optical disk drive according to claim 1, wherein saidlight guide is an optical fibre, in particular a single mode fibre, oran integrated waveguide.
 10. Optical disk drive according to claim 1,wherein said optical disk drive is a small sized optical disk drive andwherein said laser diode is adapted for generating a blue laser beam.11. Optical disk drive according to claim 1, wherein said objective ismounted on a focus control element, in particular a leaf spring, forfocus control.
 12. Optical disk drive according to claim 1, furthercomprising focus and radial tracking means, in particular integratedplanar coils, mounted on said swing arm for focussing and radialtracking of said laser beam.
 13. Detector for use in an optical diskdrive as claimed in claim 1 for detecting light reflected from theoptical disk, said detector being a split detector having at least twodetector portions.
 14. Detector according to claim 13, wherein saiddetector portions have different heights such that the distance to thesurface of said detector portions is different.